Hydrophobic, pure-silica zeolites are useful materials primarily because of their organophylic character and thermal and hydrothermal stability. Microporous, pure-silica molecular sieves can be synthesized hydrothermally using organic molecules—structure-directing agents (SDAs)—to kinetically steer their syntheses to the desired products. However, in many cases, the presence (or absence) of tetrahedral, non-silicon atoms in the synthesis such as boron, aluminum, or zinc causes the formation of different crystalline phases or prevents the formation of a crystalline phase entirely. For example, the use of the N,N,N-trimethyl-2-adamantanammonium cation as an SDA gives SSZ-24 (AFl) when boron is included in the synthesis gel, and SSZ-13, SSZ-23 or SSZ-25 when varying amounts of aluminum are included in the synthesis. In the absence of any tetrahedral, non-silicon atom in the synthesis, no crystalline products are formed. Situations like this invariably arise in zeolite synthesis and as a result, many framework topologies can only be synthesized in a narrow range of framework compositions. For example, molecular sieves of the CON topology have only been directly synthesized to date as borosilicates (SSZ-33, CIT-1) or as an aluminosilicate (SSZ-26).
To access other framework compositions, various strategies have been employed. Dealumination of zeolites is commonly carried out to synthesize high-silica or pure-silica zeolites. Many dealumination procedures have been developed over the years including steaming, treatment with mineral acids or chelating agents, reaction with silicon tetrachloride and treatment with silicon hexafluoride.
Another route to both all-silica and heteroatom-containing framework compositions is through the use of borosilicates as precursor species. Removal of boron from the framework of molecular sieves requires significantly milder conditions than does the removal of aluminum. Vacancies with tetrahedral coordination can then be repopulated in a subsequent step with a variety of species including silicon, titanium, and aluminum, among others.
Recently, the zincosilicate CIT-6 (*BEA topology) was synthesized and extensively characterized. This material is unique because it can be used as a precursor to a variety of molecular sieves of the *BEA structure. The organic SDA can be removed from the micropores by solvent extraction techniques. Furthermore, zinc, like boron, can be easily removed from the molecular sieve framework under relatively mild conditions. In particular, aqueous acetic acid treatments were found to be suitable for removal of zinc from the framework of CIT-6 while simultaneously removing the organic SDA from the micropores. Under the proper conditions, zinc can be completely removed from the material and the vacancies (defects) left behind can be healed with silicon that is presumably dissolved from other parts of the crystal. Similarly, in the development of organic-functionalized molecular sieves (OFMSs), we found that extraction of the organic SDA from the micropores of the molecular sieve using aqueous acetic acid resulted in materials that were essentially free of structural defects14. In contrast, extraction with other solvents left a material that contained a significant number of internal defects as determined by 29Si solid state NMR spectroscopy.
The present invention relates to the use of acid treatment to a variety of calcined molecular sieves with different framework compositions and structures and the generality of this methodology for preparing a broad spectrum of molecular sieve materials. Calcined borosilicate and pure-silica molecular sieves of different topologies are treated with acid under a variety of conditions and are subsequently characterized in detail. Specific attention is paid to the role of acetic acid in this system.